Radio-frequency Ar sputter etch rates: Theory and experiment

Abstract

Radio-frequency (rf) sputter etching rate calculation is difficult because ion fluxes at targets are not well known, and ion-neutral collisions in rf sheaths result in a wide range of particle energies incident on rf targets. In addition, backscattering from the sputtering gas makes experimental etch rates hard to measure. In this article, we present measurements of (a) rf sputter etching and redeposition rates, (b) Ar+ ion beam sputtering rates of Si and SiO2, and (c) ion fluxes and energy distributions for 13.56 MHz Ar discharges. A simple self-consistent model of the rf sheath is used to derive the flux and energy distribution of neutral particles. This is combined with the ion flux to derive theoretical sputtering rates. Experimentally measured etch rates have been corrected for redeposition by comparing rates on planar sufaces with rates inside reentrant masks. Low-density coatings due to redeposition were observed in regions protected from ion bombardment, inside the reentrant masks. Ar sputter etch rates of planar samples were suppressed by factors between 3 and 10 by this redeposition. Agreement between theoretical predictions and observed experimental results is good, inside the range of uncertainty in our theoretical calculations. An overall predictive accuracy within 25% is attained.